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bioRxiv · 10.64898/2026.10.20.760467

Proximal sesamoid bone fatigue in Thoroughbred racehorses is associated with palmar osteochondral disease in the adjacent third metacarpal bone condyle

Thom, E.·Irandoust, S.·Henak, C.·Muir, P.
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Abstract

Abstract Background: Proximal sesamoid bone (PSB) stress fracture is a common catastrophic injury in racing Thoroughbreds that is associated with bone sclerosis and development of focal osteolysis. Standing computed tomography (sCT) enables sensitive identification of these structural changes and associations with other more common fetlock fatigue injury such as palmar/plantar osteochondral disease lesions affecting the condyles of the third metacarpal/metatarsal (MC3/MT3). Objective: To gain insight into the PSB stress fracture mechanism, we examined associations between fatigue injury prevalence in the three major metacarpophalangeal (MCP) sites: PSB, parasagittal grooves (PSG), and third metacarpal bone MC3 condyles. Study design: Retrospective clinical study. Methods: We studied 22 catastrophically injured Thoroughbred racehorses. Thoracic limbs were examined using computed tomography (CT) (n=29). Subjective CT assessment classified 58 PSBs as 32 control and 26 case PSBs with bone fatigue (transverse fracture, focal osteolysis, or severe sclerosis). Opposing joint surfaces were also studied. PSB bone mineral density (BMD) was estimated. Results: Prevalence of fatigue injury in the PSB, condyle, and PSG was 44.8% (n=26), 43.1% (n=25), and 17.2% (n=10), respectively. For MC3 condyles with POD, 88.0% had fatigue injury in the corresponding PSB (OR = 53.2). Prevalence of PSB fatigue injury was 20.0% when there was PSG fatigue injury. The odds of a PSB having fatigue injury when the opposing MC3 PSG had signs of fatigue injury was 0.25. PSBs with opposing MC3 condyles containing a POD lesion had significantly increased BMD compared with opposing control condyles. BMD was significantly increased in PSBs with structural change associated with cyclic fatigue. Conclusions: Development of PSB fatigue and adaptive PSB sclerosis is associated with POD. Stress fractures in Thoroughbreds commonly affect the MCP joint. Condylar fractures occur from PSG or POD fatigue injury. PSB fractures are also common. Our findings suggest detailed clinical assessment of PSB structural change in racing Thoroughbreds diagnosed with POD is merited as part of routine injury prevention screening.

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Thom, E., Irandoust, S., Henak, C., Muir, P.. 2026-10-21. Proximal sesamoid bone fatigue in Thoroughbred racehorses is associated with palmar osteochondral disease in the adjacent third metacarpal bone condyle. https://doi.org/10.64898/2026.10.20.760467

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